History of Annelida

History of Annelida:

The annelids were by early zoologists included with other worms in the group Vermes but were separated by Cuvier in 1798 from the un-segmented worms. The name Annelida was first of all used by Lamarck in 1809 for the higher segmented worms.

Annelida (L., annellus = little ring or F .anneler = to arrange in rings) are triploblastic, bilaterally symmetrical, coelomate and segmented Metazoa. Body is covered by a thin cuticle having chitinous setae in most. Body wall is covered with glandular epidermis, below which are muscles forming an outer layer of circular muscles and an inner layer of longitudinal muscles.

The body is divided for the first time in the animal series into metamerically arranged segments or metameres arranged in a linear series, but the segments are integrated into a single functional unit. Perivisceral cavity is a schizocoelic coelom between two layers of mesoderm. The coelom contains a coelomic fluid which is incompressible, consequently it acts as a hydraulic skeleton.

There is a single pre-oral segment called prostomium and a similar post-segmental region posteriorly known as a pygidium. The nervous system has a pair of pre-oral ganglia or brain, and paired ventral nerve cords ganglionated in each segment. There is a closed circulatory system.

The digestive tract is more or less straight but differentiated into well-defined regions; digestion is entirely extracellular. Organs of excretion are metameric ectodermal nephridia, besides which there are tubular mesodermal coelomoducts used for passage of reproductive cells. The larva, if present, is a trochosphere.

The members of the phylum are modified for sedentary, active, or ectoparasitic life, they occur on land, in freshwater, or in the sea.

The phylum contains over 8,600 known species and is divided into four classes called Polychaeta, Oligochaeta, Himdinea, and Archiannelida. But in some systems of classification, Echiuroidea, Sipunculoidea, Priapulida, and Myzostomaria have also been included as classes, while in other systems these classes have been considered as appendix to phylum Annelida or to some of its classes.